For precision voltage reference circuits, a bootstrapped bias generator that is supplied from a sub-regulated source is often employed to provide additional supply independence and noise immunity from the input supply. Typically, a bootstrapped bias generator is connected to a circuit that generates a substantially constant output signal (Vout) from a received signal (Vin). In this type of circuit, the bias is dependent on the output signal of the circuit which is powered by that same bias.
FIG. 1 illustrates a block diagram of an exemplary reference circuit with a bootstrapped bias generator connected in a feedback loop with the circuit. During power up, the bootstrapped bias generator follows the output of the circuit (Vout) as it transitions from zero volts to its steady state value. However, since the bootstrapped (output voltage referenced) bias current follows Vout, the bias current is time dependent. Also, the circuitry in the bias loop may have more than one stable operating point. Startup circuits are employed to force a condition in which the desired operating point is converged upon. Use of such circuits often results in undesirable large-signal behavior during the time the startup circuit is engaged. Additionally, mismatch within the circuit can be a function of the bias point and can have an effect on startup. The startup behavior can also be dependent on a variable external load.
For these reasons it can be difficult to achieve robust and well damped start up behavior under all conditions using a bootstrapped bias generator alone. A non-bootstrapped (referenced from the input supply) bias generator that does not share its feedback loop with the circuit it is biasing may be employed to avoid some of the limitations of a bootstrapped bias generator. However, although an independent (input supply referenced) bias generator can bias the circuit to a known state at startup, it is more dependent on the supply (VDD) than a bootstrapped bias and can couple significant input supply noise to the operation of the reference circuit. A non-bootstrapped bias can also show some dependence on the input supply since it is referenced to it. This can translate to degraded line regulation of the system.